I suppose here is what I would look for:
1. Has a few games or games themselves or frequently cycles games. I wouldn't want the guy with 1-2 games and never sells/trades them. He may get too attached and not want to trade it back.
2. The other person shouldn't scrapped for cash. This way if the game get's scratched or whatever happens it is a bummer, but not end of the world.
3. Find someone who can easily move games in/out of their house by themselves so they can bring it to your place when its time and you can bring it to theirs. Makes it easier for both and a time saver.
4. Make it a person that if you had too many games you would be comfortable lending them a game for a few months at a time.
Obviously, you want the honest,fair etc stuff, but for pinball requirements I think those hold pretty true for me.